The invention relates generally to pulse oximeters used to detect blood oxygenation. More specifically, the invention relates to a method for calibrating a pulse oximeter and to a pulse oximeter capable of calibrating. The invention further relates to a sensor allowing the calibration of a pulse oximeter, the sensor being an integral part of the pulse oximeter.
Pulse oximetry is at present the standard of care for continuous monitoring of arterial oxygen saturation (SpO2). Pulse oximeters provide instantaneous in-vivo measurements of arterial oxygenation, and thereby an early warning of arterial hypoxemia, for example.
A pulse oximeter comprises a computerized measuring unit and a probe attached to the patient, typically to his or her finger or ear lobe. The probe includes a light source for sending an optical signal through the tissue and a photo detector for receiving the signal after transmission through the tissue. On the basis of the transmitted and received signals, light absorption by the tissue can be determined. During each cardiac cycle, light absorption by the tissue varies cyclically. During the diastolic phase, absorption is caused by venous blood, tissue, bone, and pigments, whereas during the systolic phase there is an increase in absorption, which is caused by the influx of arterial blood into the tissue. Pulse oximeters focus the measurement on this arterial blood portion by determining the difference between the peak absorption during the systolic phase and the constant absorption during the diastolic phase. Pulse oximetry is thus based on the assumption that the pulsatile component of the absorption is due to arterial blood only.
Light transmission through an ideal absorbing sample is determined by the known Lambert-Beer equation as follows:
Iout=Iinexe2x88x92xcex5DC,xe2x80x83xe2x80x83(1)
where Iin is the light intensity entering the sample, Iout is the light intensity received from the sample, D is the path length through the sample, xcex5 is the extinction coefficient of the analyte in the sample at a specific wavelength, and C is the concentration of the analyte. When Iin, D, and xcex5 are known, and Iout is measured, the concentration C can be calculated.
In pulse oximetry, in order to distinguish between two species of hemoglobin, oxyhemoglobin (HbO2), and deoxyhemoglobin (RHb), absorption must be measured at two different wavelengths, i.e. the probe includes two different light emitting diodes (LEDs). The wavelength values widely used are 660 nm (red) and 940 nm (infrared), since the said two species of hemoglobin have substantially different absorption values at these wavelengths. Each LED is illuminated in turn at a frequency which is typically several hundred Hz.
The accuracy of a pulse oximeter is affected by several factors. This is discussed briefly in the following.
Firstly, the dyshemoglobins which do not participate in oxygen transport, i.e. methemoglobin (MetHb) and carboxyhemoglobin (CoHb), absorb light at the wavelengths used in the measurement. Pulse oximeters are set up to measure oxygen saturation on the assumption that the patient""s blood composition is the same as that of a healthy, non-smoking individual. Therefore, if these species of hemoglobin are present in higher concentrations than normal, a pulse oximeter may display erroneous data.
Secondly, intravenous dyes used for diagnostic purposes may cause considerable deviation in pulse oximeter readings. However, the effect of these dyes is short-lived since the liver purifies blood efficiently.
Thirdly, coatings like nail polish may in practice impair the accuracy of a pulse oximeter, even though the absorption caused by them is constant, not pulsatile, and thus in theory it should not have an effect on the accuracy.
Fourthly, the optical signal may be degraded by both noise and motion artifacts. One source of noise is the ambient light received by the photodetector. Many solutions have been devised with the aim of minimizing or eliminating the effect on the signal of the movement of the patient, and the ability of a pulse oximeter to function correctly in the presence of patient motion depends on the design of the pulse oximeter. One way of canceling out the motion artefact is to use an extra wavelength for this purpose.
A further factor affecting the accuracy of a pulse oximeter is the method used to calibrate the pulse oximeter. Usually the calibration is based on extensive empirical studies in which an average calibration curve is determined based on a high number of persons. By means of this calibration curve, which relates the oxygen saturation of blood to pulse oximeter signals, the average difference between the theory and practice (i.e. in-vivo measurements) is taken into account. The calibration curve typically maps the measured in-vivo signal to a corresponding SPO2 value.
Pulse oximeters, however, can also utilize the Lambert-Beer model for calculating the concentrations of the different Hb species. In this method of calibration, the measurement signals must first be transformed into signals applicable to the Lambert-Beer model for calculation. This transformation constitutes the calibration of the pulse oximeter, since it is the step by means of which the in-vivo signals are adapted into the Lambert-Beer theory according to which the pulse oximeter is designed to operate. Thus, the calibration curves can also be in the form of transformations used to adapt the actual in-vivo measurements to the Lambert-Beer model. Transformations are discussed for example in U.S. Pat. No. 6,104,938.
However, each patient has a calibration curve of his or her own, which deviates from the average calibration curve calculated on the basis of a high number of patients. This is due to the fact that the characteristics of the finger of each patient, such as the absolute amount of venous blood, deviates from those of the average finger. One drawback of the current pulse oximeters is that they are incapable of taking this human variability into account. Human variability here refers to any and all factors causing patient-specific variation in the calibration curve, including time-dependent changes in the calibration curve of a single patient. As discussed in the above-mentioned U.S. Patent, patient-dependent variation can also be seen as an effect of a third substance, such as a third hemoglobin species, in the blood. However, in this context all variation is interpreted as a patient-dependent change in the calibration curve of the pulse oximeter.
It is an objective of the invention to bring about a solution by means of which the patient-specific differences can be taken into account when a pulse oximeter is calibrated. In other words, it is an object of the present invention to create a pulse oximeter which can compensate for the differences in an individual patient as compared to the average calibration or transformation curve which the current pulse oximeter relies on.
A further objective of the invention is to bring about a general-purpose solution for the calibration of pulse oximeters, a solution which is not limited to pulse oximeters explicitly using the transformations as calibration, but which can be applied to any pulse oximeter regardless of its current built-in calibration method.
These and other objectives of the invention are accomplished in accordance with the principles of the present invention by providing a mechanism by means of which a pulse oximeter can deduce, in connection with each measurement, the patient-specific deviation from an average calibration curve known to the pulse oximeter. Utilizing this difference the pulse oximeter can then determine a new, patient-specific calibration, which takes into account the individual differences to the average calibration curves. The pulse oximeter can thus adapt the calibration to the characteristics of each individual patient.
In its basic embodiment the pulse oximeter comprises three wavelengths. Two of the wavelengths are used for measuring the basic Hb species, i.e. oxyhemoglobin and deoxyhemoglobin, whereas the third wavelength is needed for the calibration method according to the invention.
In the method of the invention, so-called invariants are determined, which are parameters theoretically independent of any tissue or blood parameters, except the known extinction values. The patient-specific variation in these invariants is then used to calibrate the pulse oximeter. A theoretical value is first determined for each invariant on the basis of the average calibration curve, and then in a similar way a second value is further determined for the same invariants, except that the measured (in-vivo) signals are used instead of theoretical measurement signals. Each second value is then compared to the corresponding theoretical value and the difference(s) is/are used for calibration purposes.
The method is not limited to pulse oximeters explicitly using the transformations, but can be applied to any pulse oximeters. However, the way the above-mentioned difference(s) in the values of the invariant is/are used for calibration purposes depends on the type of the pulse oximeter. In a transformation-based pulse oximeter, a new patient-specific transformation can be searched for on the basis of the difference, the new transformation being such that it yields a minimum difference between said theoretical value and a second value determined on the basis of the new transformation itself. In a conventional pulse oximeter, which maps the measurement signal to arterial oxygen saturation, the above-mentioned difference in the values of the invariant can be mapped to an error value indicating a patient-specific divergence from an average value for arterial oxygen saturation.